The weather variables we measure and store

Atmospheric Pressure

Atmospheric pressure is the force per unit area exerted by the weight
of the air. In general it is the pressure differences in space and time
that we are interested in.
The spatial differences that we measure along the surface of the Earth give
useful information about the local wind. The day to day changes at one location
tell us what kind of
weather to expect. Usually, rising pressure brings fair weather and falling
pressure brings poor weather. We show the pressure corrected to sea level.
The units of pressure are hectopascals (hPa).
Example.

Humidity

Humidity is a measure of the amount of moisture (water vapour) in the air.
We are measuring the relative humidity. Relative humidity tells you how water
vapour is present as a fraction of the maximum amount of water vapour that
could be
present in a parcel of air at the current temperature and pressure. If the
temperature goes down and the amount of water vapour does not change then the
humidity will go up. Relative humidity is expressed in percent (%).
Example.

Rain

Rainfall is measured using a tipping bucket rain gauge that doesn't have
to be emptied. An event is recorded when about one quarter millimetre of
rain falls. Total daily rainfall is stored as a running total that is reset
to zero each day at midnight. Our rain gauges are not heated so the occasional
winter ice and snow events that we see here in Victoria will not be recorded
properly. Any snow or ice that accumulates in the bucket will be recorded when
it melts.
Example.

Incoming Solar Radiation (Insolation)

The solar radiation sensor on the weather station measures the intensity of
sunlight reaching the Earth's surface. This radiation is coming from the
entire sky, not just the sun. It is interesting to note the effect of closely
approaching clouds. Reflection of energy from clouds often causes a noticeable
peak in the strength of the intensity. The intensity of the incoming radiation
is measured in Watts per square metre (Wm-2).
Example.

Temperature

The weather station contains a temperature sensor in a special chamber
that is shielded from direct exposure to the sun. The shield is ventilated
to allow air to circulate over the sensor. Temperature is measured in degrees
Celsius (°C).
Example.

Ultraviolet (UV) Radiation

Some of the energy that comes from the sun reaches us in the form of
ultraviolet light. This kind of light is responsible for causing the skin
to burn. The UV sensor on the weather station allows us to report the UV
index during the course of the day. Here is a table of UV index values and
your risk of exposure to the sun. Human skin comes in a wide range of sun
sensitivity. Some skin types burn more quickly at a particular UV Index
than others.
Example.

UV Index

Exposure Risk

0 - 2

Low

3 - 5

Moderate

6 - 7

High

8 - 10

Very High

11+

Extreme

Wind Speed

Wind speed is measured with a cup anemometer. This device spins in the
wind in such a way that the rate of spin is proportional to the wind speed.
We store the wind speed in metres per second (ms-1) and
display it in kilometres per hour (kmhr-1). Wind speed is
variable at different timescales. We sample wind speed for a period of
one minute and store the average. With a one minute sampling interval
a lot of variation caused by eddies is observed. This makes the wind
speed observations appear "noisy". The plots show two sets of data.
The orange points show the one minute time average and the blue ones
show the maximum gust in the one minute period.
Example.

The wind speed is also available in the
Beaufort wind rose.
A wind rose is a histogram of wind directions. This plot combines the
observed directions and speeds (as Beaufort numbers). The speeds are drawn as
coloured segments of each bar in the rose.

Wind Direction

To measure the wind we need to determine both its speed and direction.
The direction is measured using a weather vane. In our case it is a simple
device that rotates to point into the wind, to indicate in what direction the
wind is coming from. We store the direction as degrees from north and report
it both in degrees and using compass headings. We plot the wind direction two ways. In the first, we show wind direction as a function of time through the day, week or month (Example).
In the second, we plot a wind rose which shows the relative amount of time the wind blows from a particular direction
(Example).

Dew Point

The dew point temperature is the temperature to which the air must cool for it to reach saturation,
for a given pressure and humidity. If the air temperature cools to the dew point, dew will form. If the dew point temperature is less than zero, frost will form.

Wind speed and
direction are represented by the black arrow with the arrow pointing to the direction that the wind is
coming from. The wind speed can be determined by the feathers on the arrow. Half a barb is 5 km/h,
a full barb is 10 km/h and a pendant is 50 km/h. In this example the wind is coming from the NW at
65 km/h.

Weather Data Sensor Specifications

Here we list the specifications of the Davis Vantage Pro2 weather
station.